The systems and methods disclosed herein are related to the art of image rendering devices such as printers and displays. Embodiments will be described in terms of laser-based electro-photographic marking engines such as are used in printers, photocopiers and facsimile machines. However, embodiments are applicable to other rendering devices, such as rendering devices that present image data in raster lines, such as display devices and other kinds of printers.
In electro-photographic marking engines, an imaging member, such as, for example, a belt or a drum, is made to carry an electrostatic charge. Portions of the imaging member are then exposed to light. The light discharges the selected portions. For example, the selected portions of the imaging member are exposed to light from a scanning laser beam or to light from one or more light-emitting diodes. For instance, light pulses from a laser are reflected off a set of mirrors that are arranged in a polygonal ring and driven to spin, thereby causing the reflected beam to scan across a portion of the imaging member. Often, the design of the marking engine is such that the reflected laser beam strikes the imaging member at an angle (as opposed to orthogonally). This geometry can mean that the reflected beam sweeps out an arc across the surface of the imaging member instead of the ideal straight line. Such an arc is a source of registration error. This kind of registration error is often referred to as bow.
For example, referring to FIG. 1, image data 106 associated with, and intended for, a target output line or raster 110 of an output image 114 can be misregistered and placed or written to locations in the output image 114 other than the intended locations 118 along the target output line or raster 110.
Another kind of registration error, often referred to as skew, can be caused by imperfections in the mounting and/or alignment of the light source or laser, scanning device or mirror relative to the imaging member. An imaging device or marking engine that includes imperfections related to skew will tend to generate images that appear to be crooked or at an unintended angle relative to the display screen or print media.
Accordingly, whatever the source of registration errors, in at least some instances, without compensation, a pixel from or associated with a target or nth raster (e.g., 110) can be written to a physical location (e.g., 122, 126, 130) on the display, imaging member and/or print medium that one would ideally expect to find data from or associated with, for example, raster number n−1 (134) or n−2 (138) and so on.
Accordingly, systems designers attempt to develop rendering devices that include as few sources of registration error as practicable. Additionally, designers try to minimize the amplitude of registration errors generated by those sources that cannot be eliminated. However, sources of noticeable registration error remain in many imaging systems. As improvements are made in other system parameters, such as, for example, resolution, remaining registration errors become more noticeable. Accordingly, attempts have been made to compensate for registration error sources through image path data manipulation. For example, contone image data is manipulated to pre-warp and/or pre-skew image data in a direction opposite and in an amplitude equal to known registration errors of a particular system. However, such contone image manipulation techniques require increased processing power or processing time and either increase the cost of systems or reduce system throughput.
Other compensation techniques work with binary or marking decision data. For example, instead of driving a laser with data from or associated with the ideal or target raster, these techniques attempt to compensate for, for example, bow or skew registration error by driving the light source (e.g., laser) with data from, for example, raster number n−1 when the laser beam is predicted to be at a point in the arc that is associated with the physical location on the imaging member that one would ideally expect to find data from raster n−1 and driving the laser with data from raster n−2 when the laser is at a point in the arc that is associated with the physical location on the imaging member that one would ideally expect to find data from raster n−2. However, these systems do not adequately address registration errors associated with sub-raster (e.g., 142, 146, 148, 152, 156, 160, 164) positioning errors or displacements, wherein positions between ideal raster positions are exposed to the light source or other imaging element. Images rendered through these sorts of compensation methods can include visually displeasing distortions.